The present invention relates to harmonic filtering devices intended for use most specifically in transmission stations equipped with high and very high power radio transmitters (over 100 kW and 1,000 kW respectively). High and very high power transmitters are generally equipped with harmonic filtering devices, beyond the anode circuits (or output circuits). The known harmonic filtering devices are reaction circuits with localised or distributed constants, associated with the transmission line between the transmitter and the load. Their function is to eliminate the harmonic signals produced at the same time as the fundamental or useful signal. These harmonic filtering devices can be of very different types, such as low pass, band pass, punctiform rejector, etc. Depending on the type of transmitter and the frequency band to be covered they are fixed or tunable. In all cases they are designed in such a way as to have at their input the rated load with correct matching to the fundamental frequency, if they are closed on the rated load impendance.
In the case of harmonic frequencies, the input impedance, which is to be as close as possible to a short circuit, is highly mismatched. The greater the filtering the greater the extent of mismatching. In this connection it is pointed out that the determination of a filtering device makes it necessary to know the load impedance, which is generally considered as having the same value for the harmonics and for the fundamental frequency in connection with the study of the filtering device. In practice this is far from being the case.
Due to the fact that there are numerous circuits behind the transmitter the load value of the filtering device varies very widely as a function of the harmonic frequency values. Bearing this in mind it is not possible to reliably guarantee a known value of the input impedance of the filtering device for the harmonic signals.
The obvious and well known consequence for designers of transmission stations is the difficulty of providing for the operation of the output stage of the transmitter in the numerous operating configurations offered by switching circuits or other variable circuits positioned downstream of the filtering device. The transmitter is not "insulated" from the load at harmonic frequencies.